Investigation of anti-proliferative compounds or cytotoxic therapeutic agents to treat malignant cells or aberrant cell proliferation is an important aspect of drug development. Viability assays that can quantify biological activity and differentiate the relative stability and toxicity of various test compounds are the most valuable. As indicated earlier, one of the more critical assumptions of such quantitative viability assays is that cell number and drug concentration share a linear and inversely proportional relationship. This assumption is violated in cases of highly variable cell number in time-course experiments because of a number of factors, for example: (i) where the cell line itself has irregular growth properties, (ii) where a smaller number of responsive non-proliferating cells is masked by a larger number of non-responsive proliferating cells, (iii) where the test compound affects cellular aggregation or adhesion-both of which can indirectly affect cell proliferation, and (iv) where the test compound causes apoptosis or changes the cell cycle which would result in a nonlinear relationship between drug dose and cell number. In some cases where feasible, it may be necessary to normalize the data to cell number by using second assay